Information and help on glass fusing, stained glass, kiln forming, acid etching, sandblasting and other areas of working with glass.

Wednesday, 19 March 2014

Annealing - Effects of Chemistry

Affects
of Chemistry on Annealing Point

The
change in the transition temperature is affected by the rate of
cooling; it is also affected by the chemistry - or composition - of
the glass. The transition temperature in silicates (glass of various
compositions) is related to the energy required to break and re-form
covalent bonds in an amorphous (or random network) lattice of the
tetrahedra form of the glass molecules.

A
covalent bond is one that involves the sharing of electron pairs
between atoms. The stable balance of attractive and repulsive forces
between atoms when they share electrons is what covalent bonding
refers to.

The
transition temperature is influenced by the chemistry of the glass.
For example, addition of elements such as Boron, Sodium, Potassium or
Calcium to a silica glass helps in breaking up the network structure,
thus reducing the transition temperatureand the melting
temperature. Alternatively, Phosphorus helps to reinforce an ordered
lattice, and thus increases the transition temperature.

The
modifiers commonly used in glass-making are: sodium oxide, potassium
oxide, lithium oxide, calcium oxide, magnesium oxide, and Lead oxide.
Although there are over 2,000 known additives to glass. The minerals
used to colour the glass seem to have minor affects upon the glass
composition as they generally are in a colloidal suspension without forming
bonds to the silica atoms.

If
an oxide, such as sodium oxide, is added to silica glass, a bond in
the network is broken and the relatively mobile sodium ion becomes a
part of the structure. With increase in the amount of modifier, the
average number of oxygen-silicon bonds forming bridges between
silicon atoms decreases. The principal effect of a modifier is to
lower the melting and working temperature by decreasing the
viscosity. An excess of modifier can make the structural units in the
melt sufficiently simple and mobile that devitrification
(crystallization) occurs in preference to the formation of a glass.
The skills of the glass makers lie in the balance of factors relating
to the transition and working temperatures, and the maintaining the
resistance to devitrification.

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